BIFURCATIONS AND MODULATIONAL INTERACTION IN NEGATIVE COMPRESSIBILITYTURBULENCE

The results of detailed numerical studies of phenomena in negative com pressibility turbulence with sheared perpendicular (i.e., poloidal) fl ow are presented. The turbulence model is based on the parallel ion fl ow gradient instability, a representative paradigm for ion drift waves . Studies of coupled turbulence and mean flow evolution indicate the e xistence of two distinct nonlinear states. In the first state, saturat ion occurs via nonlinear transfer to damped high-k modes and sheared f low is heavily damped. In the second state, the turbulence level is co ntrolled by the self-consistently generated sheared flow. Transition b etween these states is determined by the competition between instabili ty growth and damping of rotation. The dynamics of the observed transi tion is well described and consistent with a simple set of coupled env elope equations. Modulational interaction between small scale turbulen ce and large scale m not-equal 0 shear flows is observed, as well.